MUTUAL AFFINITIES OF ORGANIC BEINGS 385 



tain butterflies and moths so often assuming the dress of another 

 and quite distinct form ; why, to the perplexity of naturalists, has 

 nature condescended to the tricks of the stage? Mr. Bates has, no 

 doubt, hit on the true explanation. The mocked forms, which al- 

 ways abound in numbers, must habitually escape destruction to a 

 large extent, otherwise they could not exist in such swarms ; and a 

 large amount of evidence has now been collected, showing that 

 they are distasteful to birds and other insect-devouring animals. 

 The mocking forms, on the other hand, that inhabit the same 

 district, are comparatively rare, and belong to rare groups; hence, 

 they must suffer habitually from some danger, for otherwise, from 

 the number of eggs laid by all butterflies, they would in three or 

 four generations swarm over the whole country. Now if a member 

 of one of these persecuted and rare groups were to assume a dress 

 so like that of a well-protected species that it continually deceived 

 the practised eyes of an entomologist, it would often deceive pre- 

 daceous birds and insects, and thus often escape destruction. Mr. 

 Bates may almost be said to have actually witnessed the process 

 by which the mimickers have come so closely to resemble the 

 mimicked; for he found that some of the forms of Leptalis which 

 mimic so many other butterflies, varied in an extreme degree. In 

 one district several varieties occurred, and of these one alone re- 

 sembled, to a certain extent, the common Ithomia of the same dis- 

 trict. In another district there were two or three varieties, one of 

 which was much commoner than the others, and this closely 

 mocked another form of Ithomia. From facts of this nature, Mr. 

 Bates concludes that the Leptalis first varies; and when a variety 

 happens to resemble in some degree any common butterfly in- 

 habiting the same district, this variety, from its resemblance to a 

 flourishing and little persecuted kind, has a better chance of 

 escaping destruction from predaceous birds and insects, and is 

 consequently oftener preserved; "the less perfect degrees of re- 

 semblance being generation after generation eliminated, and only 

 the others left to propagate their kind." So that here we have an 

 excellent illustration of natural selection. 



Messrs. Wallace and Trimen have likewise described several 

 equally striking cases of imitation in the Lepidoptera of the Malay 

 Archipelago and Africa, and with some other insects. Mr. Wallace 

 has also detected one such case with birds, but we have none with 

 the larger quadrupeds. The much greater frequency of imitation 

 with insects than with other animals, is probably the consequence 

 of their small size; insects cannot defend themselves, excepting in- 

 deed the kinds furnished with a sting, and I have never heard of 



